1. Field of the Invention
The present invention relates to a control system for the axes of movement of handling machines with linear drives and/or pivoting drives for transporting and for positioning workpieces relative to and in a target point, particularly relative to and on processing machines.
2. Description of the Related Art
Handling machines of the known type as disclosed, e.g., in DE 41 36 441 C2, are used, for example, for transporting and positioning heavy forged pieces having a weight of above 100 kilograms relative to and on processing machines. Depending on the size and configuration, handling machines make it possible to position with the accuracy of a millimeter workpieces having a weight of many tons. The handling machines are equipped with a chucking device, for example, tongs, and a lever system for moving the tongs in several directions or degrees of freedom, wherein for each movement of the tongs in a certain direction a separate drive of the lever system is used. In handling machines intended for higher loads, hydraulic drives are used. Most handling machines are equipped with distance measuring systems which make it possible to determine by measuring the positions of the axes of movement.
As far as the control of the axes of movement of the handling machines is concerned, there are basically two different types of machines, namely, automatically operating machines which in accordance with a predetermined program always repeat a precisely defined sequence of movements, and manually operated machines. The first type of machines are robots if the machines have more than three degrees of freedom, wherein such robots are manufactured in various embodiments for different fields of technical applications.
The present invention is directed to handling machines which are controlled by an operator. Used as drives for the individual axes of movement of manually controlled handling machines are hydraulic drives, such as hydraulic cylinders and hydraulic motors with a decentralized hydraulic oil supply, or also electric drives.
The operator usually controls the handling machines by means of control levers which are also called joysticks. The operator requires a joystick for each available direction of movement of the machine. The directions of movement are sometimes integrated in a joystick offset by 90 degrees. For each direction of movement, the joysticks can be pivoted from the center position toward the left and right by a certain angle. Depending on the type of joystick, the joystick is returned automatically into the zero position by means of mechanical springs when the joystick is released. The joystick is coupled through a mechanical gear unit to an adjustable electric resistor whose value changes in dependence on the deflection of the joystick. By connecting a direct voltage to the resistor, the deflection of the joystick can be transformed into a voltage which is proportional to this deflection. This variable electric voltage can be utilized for two different types of control of the axes of movement of a handling machine.
In the first type of control, the variable voltage is utilized as a desired value for a speed adjustment or control. The speed adjustment or control directly influences the speed of the drive.
The operator visually controls the process, i.e., the speed and, thus, the position of the axes of movement, and is capable of controlling the speed and the position of the axes of movement in order to achieve a desired point of the displacement path of the axes of movement.
This control concept fails in the case of dynamic, quickly moveable axes of movement. When approaching a desired target position, the operator is not capable of precisely assessing the remaining distance. The axis of movement either stops in front of the desired position or it is moved past this position. It is then necessary to attempt another approach in order to achieve the desired target position. The operator has to have a certain level of experience and skill in order to be able to influence the pattern of the speed adjustment or speed control in such a way that the drive of the axes of movement comes to rest at the desired position of the axes. As a result, the operator requires a lot more time in order to accurately approach a desired target than would be necessary by the drive if based on pure computation. An advantage when using an operator as the "controller" is the high flexibility in the selection and possibilities of changing the target of an axis of movement. The above-described type of control of the axis of movement of a handling machine constitutes the classic speed adjustment. The embodiment with a speed control leads to the same result.
The second known control concept for the axes of movement of handling machines is based on a position control which can be carried out by an operator by means of a joystick.
As a rule, the joystick can be deflected from a middle zero position toward both sides by a certain angle. The extent of the maximum deflection depends on the type of construction.
The axis of movement to be controlled usually has a limited displacement distance whose magnitude depends on the type of construction of the respective handling machine. An exception are axes of movement constructed as axes of rotation or round axes which are freely rotatable in both directions of rotation.
When using a joystick for the position control of an axis of movement, the entire angle of movement of the joystick should correspond to the possible displacement distance of the driven axis of movement. Under this assumption, it is possible to assign to each position of the joystick a precisely determined position of the axis of movement on its possible displacement path. The "zero point" of the joystick is in the middle of the displacement distance of the axis of movement. For determining the relative position of an axis of movement, the axis must be equipped with a distance measuring system. This distance measuring system produces an electric signal whose magnitude is proportional to the position of the axis of movement. An adjusting signal is produced by a comparison of the position signals of the axis of movement and of the joystick. Magnitude and direction of this signal are dependent on the position of the axis of movement and the deflection of the joystick from its zero position by the operator. This position of the axis of movement constitutes the actual position and the position of the joystick results in the desired position. By carrying out a continuous comparison of the actual position and the desired position, the axis of movement is moved until the position of the axis of movement corresponds to the desired position predetermined by the joystick. This comparison is usually carried out several hundred times per second by means of an electronic system called a regulator. The speed of the regulator is far superior to the human capability to react. The axis of movement with its possible dynamics follows each new adjustment of the target position by means of the joystick by an operator.
This control principle is based on a position control. The positioning of the axis of movement to the desired target takes place more quickly and more accurately than would be the case in a speed adjustment by an operator because the positioning is exclusively determined by dynamic parameters of the axis of movement and is independent of the skill and experience of the operator. A disadvantage of this control principle is the low resolution of the position of the joystick in relation to the actual displacement distance of the axis of movement. This control principle fails especially in the case of long displacement distances of the axes of movement because, in that case, the ratio of the mechanical distances of the joystick and the axis of movement becomes too large and the desired positioning accuracy can no longer be achieved because the operator is no longer capable of positioning the joystick with sufficient precision.
The two control concepts described above have the aforementioned advantages and disadvantages and have long been used in various fields of technology.